The present invention relates to a process for coating an implant, and in particular to a process for coating a surface of a stent.
A stent is typically an open tubular structure that has a pattern (or patterns) of apertures extending from the outer surface of the stent to the lumen. It is commonplace to make stents of biocompatible metallic materials, with the patterns cut on the surface with a laser machine. The stent can be electro-polished to minimize surface irregularities since these irregularities can trigger an adverse biological response. However, stents may still stimulate foreign body reactions that result in thrombosis or restenosis. To avoid these complications, a variety of stent coatings and compositions have been proposed in the prior art literature both to reduce the incidence of these complications or other complications and restore tissue function by itself or by delivering therapeutic compound to the lumen. Difficulties in coating stents, especially electro-polished stents include the following:
1. The surface of the electro-polished stent is extremely smooth and has a mirror like surface. It is very difficult for materials to bond to this surface. These materials may include polymers, drugs, polymers encapsulated with drugs, etc.
2. The patterns or designs on the surface of the stent have several gaps or ridges in between them and while coating the surface with materials, these materials will usually fill the holes between the struts or the walls of the stent. Expansion of the stent after implantation may cause on unpredictable release of the coating agent inside the vessel wall.
3. It is desirable to have a very thin coating of material on the stent otherwise during expansion of the stent these materials will delaminate or flake off producing undesirable results.
The prior art literature discloses a number of processes and techniques that attempt to solve these and other difficulties associated with stent coating. The generally followed methods of coating stents are dip coating, spray coating, and chemical bonding.
Stents are coated by simple dip coating with a polymer or a polymer and pharmaceutical/therapeutic agents. Dip coating is usually the most successful for low viscosity coatings. The presence of pharmaceutical agents in polymers usually makes the coating solutions more viscous because they need to encapsulate the drug. Dip coating using high viscosity solutions typically causes bridging, i.e. forming of a film across the open space between structural members of the device. This bridging can interfere with the mechanical performance of the stent, such as expansion during deployment in a vessel lumen. Bridges tend to delaminate and rupture during expansion and provide sites that activate platelet deposition by creating flow disturbances in the adjacent hemodynamic environment. In addition, delamination may cause particles to dislodge from the stent surface, potentially leading to other complications. Multiple dip coatings only increase the above phenomenon and also restrict sustained release.
During a spray coating process, micro-sized spray particles are deposited on top of the stent. Particles are lost due to the atomization process and this loss also results in the loss of significant amounts of the pharmaceutical agents, which can be quite costly. In order to load the stent with a maximum drug profile for active release it is desirable to not lose as much particles as possible in the polymer matrix.
Several bonding techniques, such as anionic bonding and cationic bonding, can also be used for attaching the polymers and the encapsulated polymers on the surface of the stent. During the anionic bonding process, the polymer is applied to the surface where the bonding between the pharmaceutical agent and the polymer is a chemical mixture rather than a strong bond. In covalent bonding, the attachment of the polymer and the pharmaceutical mixture to the surface of the stent is through a chemical reaction. For example, the stent is first cleaned with a primer that leaves a hydroxyl-terminated group on the surface of the stent. This hydroxyl-terminated group attaches itself to the polymer chain, which in turn contains the pharmaceutical compound chemically attached to it. In these chemical bonding techniques, there is still a need to avoid bridging between the struts of the stent.
A number of patents have issued that attempt to address these shortcomings. For example, U.S. Pat. No. 6,273,913 issued to Wright et al. describes a stent in which rapamycin is delivered locally, particularly from an intravascular stent, directly from micropores in the stent body or mixed or bound to a polymer coating applied on the stent, to inhibit neointimal tissue proliferation and thereby prevent restenosis.
U.S. Pat. No. 6,258,121 issued to Yang et aL. discusses a stent having a polymeric coating for controllably releasing an included active agent. The polymeric coating includes a blend of a first polymeric material, which if alone, would release the agent at a first, higher rate, and a second polymeric material, which if alone would release the agent at a second, lower rate over a longer time period. One stent coating utilizes a faster releasing hydrophilic polymeric material and a slower releasing hydrophobic material.
U.S. Pat. No. 6,251,136 issued to Guruwaiya et al. describes a pharmacological agent that is applied to a stent in dry, micronized form over a sticky base coating. A membrane forming polymer, selected for its ability to allow the diffusion of the pharmacological agent therethrough, is applied over the entire stent. More specifically, a stent, typically a metal stent has a layer of sticky material applied to selected surfaces of the stent. A pharmacological agent is layered on the sticky material and a membrane forming a polymer coating is applied over the pharmacological agent. The membrane is formed from a polymer that permits diffusion of the pharmacological agent over a predetermined time period.
U.S. Pat. No. 6,248,127 issued to Shah et al. describes coatings in which biopolymers may be covalently linked to a substrate. Such biopolymers include those that impart thrombo-resistance and/or biocompatibility to the substrate, which may be a medical device. The disclosed coatings include those that permit coating of a medical device in a single layer, including coatings that permit applying the single layer without a primer.
U.S. Pat. No. 6,231,600 issued to Zhong describes a device, such as a stent, which is provided with a hybrid coating that includes a time released, restenosis inhibiting coating and a nonthrombogenic coating to prevent clotting on the device. One first coat or layer includes a polymer, a cross linking agent, and pacitaxel, analogues, or derivatives thereof. A stent can be provided with a first coat including an aqueous dispersion or emulsion of a polymer and an excess of cross linking agent. The first coating can be dried, leaving a water insoluble polymer coating. A second aqueous coating including a solution or dispersion of heparin can be applied over the first coating, the heparin becoming covalently bound to the cross linking agent on the first coating surface.
U.S. Pat. No. 6,203,551 issued to Wu describes a chamber that allows a user to medicate an implantable prosthesis such as a stent. The implantable prosthesis is capable of securing a therapeutic substance and subsequently delivering the therapeutic substance to local tissues. The chamber allows a user to medicate the prosthesis subsequent to the sterilization process and immediately prior to the implantation procedure. The chamber includes a prosthesis crimped on a balloon of a catheter assembly. A user can supply therapeutic substances into the chamber and allow the therapeutic substances to be secured by the prosthesis. After allowing the prosthesis to be soaked by the therapeutic substances for a predetermined amount of time, the chamber is removed and the prosthesis is ready for the implantation procedure.
U.S. Pat. No. 6,153,252 issued to Hossainy et al. describes a process that attempts to avoid bridging. The stent is contacted with a liquid coating solution containing a film forming biocompatible polymer under conditions suitable to allow the film forming biocompatible polymer to coat at least one surface of the stent while maintaining a fluid flow through the passages sufficient to prevent the film forming biocompatible polymer from substantially blocking the passages. The patent also described stents coated by this process.
U.S. Pat. No. 6,071,305 issued to Brown et al. relates to a directional drug delivery stent that includes an elongated or tubular member having a cavity containing a biologically active agent. In one embodiment, the active agent is diffused from the reservoir directly to the walls of a body lumen, such as a blood vessel, through directional delivery openings arranged on an outer surface of the elongated member. Another variation of the stent includes an osmotic engine assembly for controlling the delivery of the active agent from the reservoir.
U.S. Pat. No. 5,891,507 issued to Jayaraman describes a process whereby the stent is dipped inside a bath of the coating material. The stent oscillates inside the bath with application of external energy. Ultrasonic energy is usually applied externally, which permits the rotation and vibration of the stent while it is immersed in the medium.
U.S. Pat. No. 5,837,313 issued to Ding et al. describes a method of coating an open lattice metallic stent prosthesis which includes sequentially applying a plurality of relatively thin outer layers of a coating composition comprising a solvent mixture of uncured polymeric silicone material and cross linked and finely divided, biologically active species to form a coating on each stent surface. The coatings are cured in situ and the coated, cured prosthesis are sterilized in a step that includes preferred pretreatment with argon gas plasma and exposure to gamma radiation electron beam, ethylene oxide, or steam.
Published U.S. patent application Ser. No. US2001/0027340 describes delivery of rapamycin locally, particularly from an intravascular stent, directly from micropores in the stent body or mixed or bound to a polymer coating applied on stent, to inhibit neointimal tissue proliferation and thereby prevent restenosis.
In spite of this prior art, a need still exists for an improved process for coating a surface of a stent.
Stents typically have a lumen, inner and outer surfaces, and openings extending from the outer surface to the inner surface. The present invention relates to a method for coating a surface of a stent. At least a portion of the stent is placed in contact with a coating solution containing a coating material to be deposited on the surface of the stent. A thread is inserted through the lumen of the stent, and relative motion between the stent and the thread is produced to substantially remove coating material within the openings.
The thread can have a diameter substantially smaller than the diameter of the lumen. The thread can be inserted through the lumen either after or prior to contacting the stent with the coating solution. Relative motion between the stent and the thread can be produced prior to contacting the stent with the coating solution to clean the stent. The thread can be either a filament or a cable with a plurality of wires. The thread can be made of a metallic or polymeric material.
The stent can be dipped into the coating solution or spray coated with the coating solution. The coating material can include a biocompatible polymer, either with or without a pharmaceutically active compound.
In one embodiment, the relative motion is oscillatory motion produced by a vibrating device. The oscillations can be changed (magnitude and/or frequency) to vary thickness of the coating solution on the stent. In another embodiment, the relative motion is produced by a shaker table. Regardless of the type of motion, the relative motion can be produced either after or while the stent is in contact with the coating solution.
The relative motion can be motion of the stent along the thread. For example, a first end of the thread can be attached to a first stand at a first height and a second end of the thread is attached to a second stand at a second height. The relative motion is produced by a gravity gradient, with the first height differing from the second height. Furthermore, the stent can be moved back and forth between the first and second stands by sequentially increasing or decreasing at least one of the first and second heights. In this way, multiple coatings can be applied to the stent.